1. Field of the Invention
The present invention relates to a document reading apparatus. More particularly, the present invention relates to a document reading apparatus to be used as a scanner section for reading a document image on a sheet (paper sheet) in a facsimile apparatus, a printer, an electrophotographic copying machine, or the like. Especially, the present invention relates to a document reading apparatus in which dispersion (variance) of a starting position of reading a document sheet at a reading section is reduced and poor sheet transporting is prevented.
2. Description of the Related Arts
For example, as a feed mechanism constructed in a scanner section of a transmitting system of a facsimile apparatus, a simplified sheet feeding mechanism is often in practical use. In the simplified feed mechanism, one feed roller is disposed in a feed section and one transport roller performs functions both as a sheet guiding section which forwards to a reading section a document sheet supplied from a feed section and as a paper discharging section which discharges the document sheet after the document sheet is read.
As shown in FIG. 5, a conventional document reading apparatus equipped with a simplified feed mechanism includes, for example, a feed section 21 for feeding a document sheet, a sheet discharging section 22 disposed downstream of the feed section 21 in a feed direction, a sheet detecting section (sheet sensor 23) disposed between the feed section 21 and the sheet discharging section 22, and a reading section 24. The reference symbol L represents a sheet path and R.P. represents a reading position.
The feed section 21 is provided with a feed roller 25, a feed spring 26 for allowing the document sheet to fit on the feed roller 25, a separating rubber plate 27 for separating a plurality of document sheets, and a separating press plate 28 for pressing the separating rubber plate 27. The sheet sensor 23 detects a front edge of the document sheet upstream of the reading section 24. The reading section 24 is provided with an optical system unit 29 for reading the document sheet. The sheet discharging section 22 is provided with a transport roller 30 and a pinch roller 31 for pressing the transport roller 30. Further, a sheet tray 32 is mounted upstream of the feed roller 25; and a sheet guide 33 is mounted between the feed roller 25 and the optical system unit 29.
In the above construction, document sheets transported from the sheet tray 32 to the feed roller 25 are separated midway by the separating rubber plate 27 and, at the same time, a separated document sheet is guided to fit on the feed roller 25 by being pressed by the feed spring 26. At this time, the feed roller 25 is rotating at a constant speed in a direction shown by arrow (a), and the document sheets are transported to the reading section 24 one by one by the rotation of the feed roller 25.
Midway therebetween, the sheet sensor 23 detects the document sheet and, upon receiving a detection signal from the sheet sensor 23, the optical system unit 29 starts a reading operation. The front edge of the document sheet which has passed the reading section 24 is nipped between the transport roller 30 and the pinch roller 31 and, thereafter, the document sheet is transported in such a manner that the transport roller 30 receives the document sheet from the feed roller 25. Subsequently, the document sheet which has finished the reading operation is discharged by the transport roller 30.
FIG. 6 shows another prior art example of a document reading apparatus. The simplified feed mechanism shown in FIG. 6 is constructed generally in the same manner as in the above-described example except that the optical system unit 29 of the reading section 24 is disposed to face the transport roller 30. By this construction, the document sheet forwarded to the transport roller 30 is nipped between the transport roller 30 and the optical system unit 29, so that the document sheet is read by the optical system unit 29 at a position of contact with the transport roller 30.
On the other hand, Japanese Unexamined Patent Publication No. HEI 4(1992)-32429 discloses a feed mechanism for preventing feed of overlying transcription sheets. This mechanism is provided with a drive roller 131 and a pinch roller 132 disposed, between a feed roller pair (not shown) and a resist roller pair (not shown), at a position downstream of the sensor 141 detecting the transcription sheet, as shown in FIG. 7. Here, the pinch roller 132 is provided with a torque limiter comprising a spring pair 137 for urging the pinch roller 132 onto the drive roller 131, a hub 140 for allowing the pinch roller 132 to rotate in the same rotation direction as the drive roller 131, and a spring 139.
When only one transcription sheet is inserted between the drive roller 131 and the pinch roller 132, the torque limiter is actuated and the pinch roller 132 is rotated in accordance with the rotation of the drive roller 131 by a normal urging force of the spring 137. However, if a plurality of transcription sheets are inserted between the drive roller 131 and the pinch roller 132, the urging force of the spring 139 increases to release the torque limiter and the hub 140 drives the pinch roller 132 to rotate in the same direction as the drive roller 131 and, at the same time, the urging force of the spring pair 137 decreases, whereby only one transcription sheet which is in contact with the drive roller 131 is transported by the drive roller 131.
The above-mentioned prior art document reading apparatus shown in FIGS. 5 to 7 have the following drawbacks.
Namely, in feeding a document sheet to the optical system unit 29, the document sheet is transported between the feed roller 25 and the transport roller 30 only by the rotation operation of the feed roller 25 and the pressure of the separating rubber plate 27 which is pressed by the separating press plate 28. Therefore, a dispersion in the feed accuracy occurs by a combination of a condition of the document sheet based on a difference in the thickness and the surface friction coefficient, an environmental condition such as an ambient temperature and humidity, a dispersion in the pressing weight imposed upon the document sheet, and the like.
Accordingly, there is a problem that a dispersion of a starting position of reading the front edge of the document sheet occurs between the sheet sensor 23 and reading position of the optical system unit 29 due to a dispersion of the feed accuracy (amount of feeding the document sheet). Also, there is a fear that a reading elongation of the document sheet occurs at the downstream thereof between the reading position of the optical system unit 29 and the transport roller 30 due to poor feeding of sheet such as slippage.
On the other hand, the construction of the feed mechanism section shown in FIG. 7 provides a mechanism such that, when a plurality of overlying transcription sheets are transported, only one of the transcription sheets is sent out by means of the torque limiter operating in accordance with a balance of urging forces between the spring 137 and the spring 139. Here, the urging force of the spring 137 urging the pinch roller 132 onto the drive roller 131 changes when the urging force of the spring 139 changes. Therefore, as in the above-mentioned prior art, it is difficult to prevent generation of a dispersion in the starting position of reading the front edge of the document sheet and an elongation of the document sheet due to the dispersion in the feed accuracy with respect to a transcription sheet inserted between the pinch roller 132 and the drive roller 131.